When you ‘find out what you’re made of’ or ‘show what you’re made of’ it usually means to demonstrate an ability to manage a difficult situation or to prove your abilities or skills in extreme circumstances. This idiom refers to the very core characteristics of an individual and what they are able to cope with. This is a casual expression. 

1. The final tomorrow is where I'll find out what I'm really made of. I hope I prove to be good enough.

1. I'm still not sure about my new staff members, but tonight I'll be finding out what they're made of as we're fully booked and will be totally run off our feet. 
2. The big sales pitch tomorrow will be the perfect opportunity for me to show my business partners what I'm made of. I have to make sure I nail it!